Many structural members are formed of precast structural concrete including building wall panels. Such panels may vary in weight from a few pounds to several tons and may be precast in a mechanized central plant or by somewhat cruder methods right on the building site. Regardless of where the precasting takes place, it is important that the process permit production controls and the use of high quality concretes and that repeated use of forms be permitted.
In wall panel applications the concrete frequently serves not only as a load bearer but in an ornamental or aesthetic sense. In such use the panel is generally reinforced by steel bars embedded in the concrete, with the tensile strength of the steel supplementing the compressive strength of the concrete.
One manner of reinforcing concrete is by placing it in a state of permanent compression through prestressing. A method of prestressing or pretensioning concrete panels or slabs is to stretch steel bars or the like between two abutments in a form before the concrete is placed. After the concrete has hardened around the tensioned bars the stretching forces are released as by cutting the ends of the bar and, as the steel seeks to regain its original length, the tensile strength in the steel is translated into compressive stress in the concrete by the bond between the steel and concrete.
Another method of reinforcing concrete is accomplished through a post tensioning process. Here steel cables or wires are located in ducts embedded in the concrete. After the concrete has hardened the steel is stretched by hydraulic rams and fastened by wedges or other gripping devices. In this process the steel is tensioned against the concrete so that the tension exerted on the steel corresponds to an equal compression on the concrete.
There are three principal steps in the manufacture of a concrete masonry unit. These are (1) the proper mixing of the cement, water and aggregates, (2) the molding of the units, and (3) the curing or drying. Generally the molding is by tamping the concrete into the molds or by vibrating or pressure.
It is desirable that building blocks or tiles be provided with an exterior surface having ornamental characteristics which are formed at the time of casting of the block. The present invention is directed generally to a form construction for casting building wall panels or blocks of cementitious material which provide for simultaneous casting of two such panels and means for separating such panels after the material is set and prior to removal from the form.
The prior art discloses the forming of building blocks or tiles with frangible interior wall portions which may be severed to divide the block into two or more sections each of which may be used for a different purpose or to present a different appearance in a building wall. As exemplary of the disclosure of such blocks and in some cases the molds or forms therefor are U.S. Pat. Nos. 2,560,731 to Miller; 1,501,709 to Grueby; 1,534,353 to Besser; 787,199 to Lloyd; and 1,980,270 to Harkort.
While these patents show blocks which are severed in some way after molding the severing or splitting is generally accomplished by some mechanical wedging means or the like which requires considerable skill and which can result in loss of the block when the splitting is imperfectly performed. In the present invention the possibility of human error is reduced considerably by providing a weakening plane in the block during the molding or casting thereof and then applying contraction or tension pressures to the block on opposite sides of the weakening plane to cause the block to split along said plane. A block so formed with a weakening plane could, of course, also be split mechanically with or without vibration.
Preferably the weakening plane is formed by a series of elongated tubes or inserts positioned in spaced parallel, side by side relation along the plane and adapted to give a desirable ornamental surface on both sections split on the plane to serve as ornamental interior or exterior surfaces for a building wall.